This invention relates to an antenna apparatus used for mobile communication equipment and, particularly, to a wide-band antenna apparatus having a wide frequency band, such as an on-vehicle cellular antenna apparatus.
A wide-band antenna apparatus of this kind is one having, for example, a transmission/reception frequency band of 824 MHz to 894 MHz, and a frequency band width of 70 MHz. An on-vehicle cellular antenna apparatus is mounted inside the vehicle, such as inside the dashboard or inside the vehicle body. Therefore, the on-vehicle cellular antenna apparatus must be one of the type of a low profile or of a planar type instead of the antenna apparatus of the pole type which is generally used.
As the antenna apparatus of the low profile type, there has been widely known an antenna apparatus called inverse F-type antenna apparatus (for example, Japanese Unexamined Patent Application Publications Nos. JP-A-8-78943 and JP-A-8-250925).
A conventional inverse F-type antenna apparatus 10 will now be described with reference to FIGS. 1 to 4. FIGS. 1 and 2 are a perspective view and a plan view of the inverse F-type antenna apparatus 10, and FIGS. 3 and 4 are a front view and a right side view of the inverse F-type antenna apparatus 10.
The inverse F-type antenna apparatus 10 includes a grounding conductor 12, an L-shaped radiating conductor 14, and a vertical conductor 16.
In detail, the grounding conductor 12 is of a square shape having a side of a length WG. In the illustrated embodiment, the grounding conductor 12 has a length WG of 90 mm. The radiating conductor 14 includes a vertical portion 141 extending vertically from a feeding point 18 provided maintaining a very narrow gap to the grounding conductor 12, and a horizontal portion 142 extending in parallel with the grounding conductor 12 from an end (upper end) of the vertical portion 141. The vertical portion 141 has an inverse isosceles triangular shape with the feeding point 18 as a vertex. The sides of the inverse isosceles triangle opposing the vertex are forming the end (upper end) of the vertical portion 141. The horizontal portion 142 is of a rectangular shape having a length LL and a width WL. In the illustrated embodiment, the horizontal portion 142 has a length LL of 69.75 mm and a width WL of 30 mm. An end of the horizontal portion 142 is connected to the end (upper end) of the vertical portion 141, and the other end of the horizontal portion 142 is opened. The length from the feeding point 18 of the radiating conductor 14 to the open end is selected to possess an electric length of about one-fourth the radiation wavelength.
The vertical conductor 16 has a rectangular shape and is located at a position slightly separated from the vertical portion 141. The vertical conductor 16 is vertically extending in parallel with the vertical portion 141 of the radiating conductor 14 from the grounding conductor 12 to the horizontal portion 142. That is, one end of the vertical conductor 16 is connected to the grounding conductor 12, and the other end of the vertical conductor 16 is connected to the horizontal portion 142 of the radiating conductor 14. The vertical conductor 16 is also called short-circuiting conductor. In the illustrated embodiment, the vertical conductor 16 has a height HL of 34 mm. The height HL of the vertical conductor 16 is nearly equal to the height of the inverse F-type antenna apparatus 10.
A coaxial cable 20 is connected to the inverse F-type antenna apparatus 10. As is well known, the coaxial cable 20 has a center conductor and an outer conductor. The center conductor of the coaxial cable 20 is electrically connected to the feeding point 18, and the outer conductor of the coaxial cable 20 is electrically connected to the grounding conductor 12.
A combination of the L-shaped radiating conductor 14 and the vertical conductor 16 is called an inverse F-element. As shown in FIGS. 1 and 2, the inverse F-element is provided at an end of the grounding conductor 12 instead of at the center of the grounding conductor 12. This is because, if the inverse F-element is brought to an end of the grounding conductor 12, the current profile varies due to the interaction between the grounding conductor and the inverse F-element, and the impedance matching can be easily selected.
The above-mentioned inverse F-type antenna apparatus 10 can be realized in a low profile. However, it has been desired to further decrease the height yet maintaining the antenna characteristics such as the radiation pattern characteristic.